This project focuses on the interactions between experience and adult neurogenesis in the hippocampal region of the brain. We are interested in understanding how experiences, including positive and stressful experiences, regulate adult neurogenesis and how the new neurons alter responses in these situations. We study the regulation and function of adult neurogenesis in rats and mice, which show continued production of new neurons throughout adulthood similar to that in primates, including humans. We have previously found that specifically inhibiting adult neurogenesis in mice increases their response to stress, leading to enhanced caution, or anxiety-like behavior, in unpredictable threat situation. During the past year, we have asked whether adult neurogenesis affects behavior under positive, or rewarding, conditions as well as stressful, threatening conditions. To do this, we used transgenic rat and mouse lines that we developed that allow us to stop neurogenesis with no side effects by feeding the animals an antiviral drug. We trained these animals lacking adult neurogenesis, and sibling control animals with normal adult neurogenesis, to lever press for food rewards. We found that loss of new neurons had no effect on the animals ability to learn to lever press for rewards or on their preference for one reward over another. However, when animals had to work progressively harder for each successive reward, the animals lacking adult neurogenesis gave up sooner. And when animals could eat plain chow or work for rewarding food, the animals without new neurons earned fewer rewards. These effects were seen in both mice and rats. However, these effects were not observed when a highly preferred food reward was used in testing. Taken together, these findings suggest that adult neurogenesis does affect behavior under non-stressful, rewarding conditions. Specifically, the new neurons appear to enhance motivation to work for weak, but not strong, rewards. This phenotype appears similar to a human condition described as behavioral apathy.